High pressure injection molding devices, e.g. those devices operating at injection pressures of greater than 1000 psi, are well-known in the art for their use in producing plastic components. In a conventional injection molding machine, a mold sized for the machine must be properly positioned in order to receive plastic through a high temperature/pressure injection process. In this manner, the mold is placed within the machine by first opening a mold clamp section wherein the mold can be mounted to a front platen of the machine. This mounting is usually performed by the use of clamps bolted to some, of the many, threaded holes in the platen. The back half of the clamp section must also be set, which is a complicated adjustment, for the clamp section must be firm but not too tight if proper plastic flow is expected. Knock-out bars, referred to as ejectors, must also be adjusted for ejection of the finished parts. The ejectors are positioned for proper length of travel to eject the finished parts. It is critical that the ejectors do not over travel, or mold damage will occur. When the adjustment is complete, the clamp section is closed to secure the back half of the mold to the rear platen, again typically with clamps. The mold may then be cycled open and closed to permit ejector and clamp pressure adjustment. Upon positioning of the movable portion of the mold to its mating position with the stationary mold half, sufficient clamping pressure must be externally applied and maintained during high pressure injection to prevent flashing of molten plastic from the mold interface, and to prevent warpage during the cool-down and shrinkage phase, prior to ejection of the parts. Maintenance of the required clamping pressure is normally maintained by applying sufficient external force, e.g. via the use of a hydraulically powered ram, so as to oppose the internal pressures developed within the mold cavity during injection. Mold speed is set to occur within a cycle specified.
Heater zones must be turned on, usually three to six, depending on the machine size. Temperatures must be set according to the plastic material being used wherein variations run from 300 degrees Fahrenheit to 700 degrees Fahrenheit. If the temperature is too hot, the plastic will burn, and if it is too cold, damage to the machine will likely occur. The controllers on the machine regulate and maintain set temperatures within a very close range at a very considerable cost. While a machine is heated up, water lines on the mold are installed and tested for leaks.
Once all heat zones are stabilized, the injection unit is retracted from the mold area and materials added to the hopper. Molten plastic is then extruded from the nozzle to remove contaminated plastic that was used previously in the machine. This can be time consuming and materially wasteful, the amount of wasted material varies dependent upon the specific type of plastic and color of plastic selected. For example, if the machine previously had black color, and the new material is clear, it is not uncommon to use up to 100 pounds of plastic prior to making the first acceptable molding.
In operation, a shot size is determined and set, usually by moving limit switches located on the rear of the injection unit. Too much plastic will make the mold flash open, and too little will cause ejection problems. Estimates can be problematic, owing to the discrepancies caused by other variables such as pressure, temperature of the plastic, and back pressure. If the weight of the part to be formed is known, air shots can be made and weighed, otherwise the operation is guesswork. Once the settings are made manually, the machine timers must be set for a semi-automatic or automatic cycle. This requires trial and error but in either event, a trained set-up man can still spend several hours getting a machine on cycle, making acceptable parts, and still the operator can change any number of controls in seconds to make inferior parts that are not immediately identified.
The above complications are multiplied when additional molds are used. For instance, if ten molding machines are employed, the above set-up must be repeated ten times. In addition, when one mold machine is being set-up or serviced, the plastic is allowed to stagnate, if not cool, causing the malfunction of the plastic feeder and/or injector system. This non-operation can cause problems in and of itself.
What is lacking in the art is a compact high pressure injection molding device of simplified design, which maintains nominal pressure upon the mold cavity, prior to and during high pressure injection molding, while eliminating the need for additional means for generating and/or maintaining externally applied clamping pressure forces, e.g. hydraulic rams and the like; and wherein all process functions are commonly controlled from a single source.